1. Field of the Invention
The present invention relates to a method for controlling regenerative braking of an electric vehicle and, more particularly, to a method for controlling regenerative braking of an electric vehicle that compensates the difference between slip amounts of left and right wheels when an anti-lock braking system ABS is activated in a regenerative braking mode.
2. Description of Related Art
In general, an electric vehicle is a vehicle that drives a motor using electric energy stored in a battery and uses the driving force of the motor as a power source wholly or partially. At present, the electric vehicle is classified into a pure electric vehicle that applies only the electric energy of the battery as the power source and a hybrid electric vehicle that includes an engine to apply the power generated from the engine to the battery charge and/or to the driving of the vehicle.
In general, an electric vehicle in braking applies a portion of braking force to the generation of electric power and uses the generated electric energy in charging the battery. Here, a portion of the kinetic energy is used as energy required for driving an electric generator, thus realizing the reduction of the kinetic energy (i.e., the reduction of the driving speed) and the generation of electric energy at that same time. This braking method is called the regenerative braking.
The generation of the electric energy in the regenerative braking mode is achieved by a separate electric generator or by driving the motor reversely. By the regenerative braking control in braking the electric vehicle, the available driving distance of the electric vehicle is increased. In case of the hybrid electric vehicle, fuel efficiency is improved and the generation of noxious exhaust gas is reduced as well.
A hydraulic brake system that causes a braking force by hydraulic pressure may be equipped in the electric vehicle However, such a hydraulic brake system may not attain a sufficient braking force only with the regenerative braking force, since desirable vehicle dynamics control cannot be obtained only with the braking force of the driving wheels as the regenerative braking force is generated in the driving wheels connected to the motor.
According to a conventional regenerative braking control method and apparatus, the regenerative braking control is performed in addition to the hydraulic braking force produced as the brake pedal is operated by a driver. The regenerative braking force is controlled in a quantity that is not related to the magnitude of the total braking force required or the magnitude of the hydraulic braking force. Thus, the braking force (the operation of the brake pedal) that the driver wants is made of the sum total of the hydraulic braking force and the regenerative braking force. Accordingly, if a desired braking force is not obtained only with the regenerative braking force, the hydraulic braking force that generates thermal energy by the frictional force of a disc and a pad is applied as much as its difference, and the regenerative braking force operates the electric generator when reducing the speed of the driving motor dedicated to the drive to generate the necessary braking force and, at the same time, the electric power generated at this time is stored in the battery.
As can be found from the graph of FIG. 5 depicting the changes in a hydraulic braking torque and a regenerative braking torque in the regenerative braking mode, the regenerative braking torque is generated as soon as the braking is started and the hydraulic braking torque by the hydraulic control is added thereto, thus generating a braking force that the driver wants.
However, conventional regenerative braking systems have drawbacks in that there is no control for compensating the difference between slip amounts of left and right wheels when the ABS is activated in the regenerative braking mode, thus generating the unnecessary hydraulic braking torque and regenerative braking torque.